1. Field of the Invention
The present invention relates to semiconductor devices such as DRAMs and nonvolatile RAMs, and piezoelectric, pyroelectric and optoelectric devices using ferroelectric ceramics such as lead zirconate titanate (PZT) or lanthanum lead zirconate titanate (PLZT) in an insulating layer, and a method of manufacturing the same.
2. Description of the Background Art
Ferroelectric ceramics such as PbTiO.sub.3, PZT and PLZT are utilized in piezoelectric, pyroelectric and optoelectric devices and the like. The application thereof ranges widely over the fields of oscillators, filters, infrared sensors and the like.
In recent years, attention is directed to ferromagnetic ceramics as a dielectric material for forming the capacitance of DRAMs and nonvolatile RAMs. These ceramics have applicability to nonvolatile semiconductor memories due to the fact that once voltage is applied utilizing ferroelectricity, the data can be maintained even if the voltage is removed. The potential of these materials for application to memory cells in semiconductor memory devices of high density is great since ferroelectric ceramics with significant high dielectric constant can store a great amount of charge even in capacitors having high integration density and small electrode area.
FIG. 1 shows a conventional DRAM using ferroelectric ceramics as the dielectric of a stacked type capacitor. Referring to FIG. 1, the conventional DRAM has an isolation oxide film 132 for element isolation formed at a predetermined region on the main surface of a silicon semiconductor substrate 131. A channel stopper layer 133 is formed beneath isolation oxide film 132. In the region surrounded by isolation oxide film 132, source/drain regions 134 and 135 with a predetermined distance therebetween are formed so as to sandwich a channel region 136. A gate electrode 138 is formed on channel region 136 with a gate insulating film 137 therebetween. An insulating film 139 is formed so as to cover gate electrode 138. A buried bit line 140 electrically connected to source/drain region 134 is formed so as to extend along the surface of insulating film 139. An interlayer insulating film 141 having a contact hole 141a is formed on source/drain region 135. An interconnection layer 142 of polysilicon is formed in contact hole 141a so as to electrically connect source/drain region 135. A platinum layer 143 is formed on interconnection layer 142 and extending above interlayer insulating film 141. A ferroelectric film 144 such as of PZT or PLZT is formed on platinum layer 143. A capacitor upper electrode 145 such as of platinum is formed on ferroelectric film 144. An interlayer insulating film 146 having a contact hole 146a is formed on capacitor upper electrode 145. An interconnection layer 147 is formed so as to extend on interlayer insulating film 146 and to be electrically connected to capacitor upper electrode 145. In such a structure, a capacitor lower electrode is formed of interconnection layer 142 and platinum layer 143.
FIG. 2 shows the hysterisis characteristics of a capacitor using ferroelectric ceramics. When a voltage of Vcc/2 or -Vcc/2 is once applied across the electrodes of the capacitor, charge of Qr or -Qr can be stored even after the voltage is removed.
A possible consideration is that the ferroelectric ceramics can be substituted by the gate oxide film of a MOS field effect transistor (FET). For example, in the FET shown in FIG. 3, a gate insulating film 110 is formed of ferroelectric ceramics to form the circuit shown in FIG. 4. When a voltage signal shown in FIG. 5 (A) is applied to the IN terminal of the circuit shown in FIG. 4, it is expected that a voltage signal as shown in FIG. 5 (B) appears. More specifically, when a positive or negative voltage is once applied to the gate electrode, the FET is switched ON/OFF, whereby data can be maintained even after voltage is removed. Such a device is called a MFSFET (Metal-Ferroelectric-Semiconductor FET) with potential of applicability to a nonvolatile semiconductor memory device.
In application of ferroelectric ceramics to the above-described electric devices, a technique for forming a thin film of ferroelectric ceramics is indispensable. A sputtering method has conventionally being employed in formation of such a thin film. This method, however, has disadvantages such as low deposition rate, generation of surface defects, and difficulties in stoichiometric control in films.
Japanese Patent Laying-Open No. 59-42392 discloses a method of forming a precursor of ferroelectric ceramics including lead in a liquid phase. In this method, a compound represented by a general formula of Pb (OCOR).sub.2 is used as a source material of Pb, and zirconium alkoxide and titanium alkoxide are used as the source materials of zirconium and titanium, respectively. When these source materials are heated and mixed together, a precursor of ceramics is produced. By applying an organic solvent solution of the precursor to a substrate, followed by a heating step, a thin film of ceramics is produced.
Japanese Patent Laying-Open No. 2-6335 discloses a method of forming a thin film of PZT by means of a sol-gel method. According to this method, a mixture consisting essentially of lead alkoxide or lead acetate, zirconium alkoxide, titanium alkoxide, ethanolamine and alcohol is applied onto a substrate and then dried to form a gel. By sintering the gelled mixture, a PZT thin film is produced on the substrate. Such a liquid phase method has the advantage of easy manipulation and allows stoichiometric control of a film. Moreover, the film thickness can be varied by changing the concentration of the mixture to be applied on the substrate or the number of times the mixture is applied. However, this method has the disadvantage of difficulty in forming a relatively thin film (for example, a thickness less than 2000 .ANG.). Furthermore, an adequate step coverage may not be achieved, resulting in a rough surface of deposited layers.
A chemical vapor deposition (CVD) method is effective for solving the above-described disadvantages. For example, the formation of a PbTiO.sub.3 thin film has been reported (Technical Papers of the Ceramics Society of Japan 96 [6] 687-93 (1988)) by a MOCVD method using an organometal compound as a source material. According to this method, tetraethyl lead (referred to as PbEt hereinafter) and titanium isopropoxide (referred to as i-POT hereinafter) were used as the source materials of Pb and Ti, respectively. As a substrate for precipitation of a dielectric thin film, monocrystals of mirror polished Si (100), sapphire (0001) and MgO (100) were used. FIG. 6 schematically shows the reported CVD device. i-POT and PbEt stored in bubblers 91 and 92, respectively, were introduced into a reaction chamber 97 by N.sub.2 carrier gas via a pipe 93b heated by a ribbon heater 93a. Reaction chamber 97 was evacuated by a rotary pump 96. The mixed source gas was blown from a nozzle 98 onto a substrate 94 heated by a heater 95. A PbTiO.sub.3 film was obtained at the substrate temperature of 500.degree. C.-600.degree. C.
The formation of a PZT thin film using a MOCVD method similar to the above-described method has also been reported (Japanese Journal of Applied Physics Vol. 29, No. 4, April 1990, pp. 718-722). According to this method, zirconium tetraisopropoxide (referred to as POZ hereinafter) or zirconium tetra-dipivaloylmethane (referred to as Zr (DPM).sub.4 hereinafter) was employed as a source material of zirconium besides the above-described source materials of Pb and Ti. As source material systems, two systems of PbET-POZ-POT and PbET-Zr(DPM).sub.4 -POT were tested. As a substrate for forming a PZT thin film, monocrystals of mirror polished Si (100) and MgO (100) were used. In the respective types, the three source materials were carried and mixed by a carrier gas at the rate of 40-100 ml/min. In response to the mixed source gas on the substrate heated to a temperature of 500.degree. C.-650.degree. C., a PZT thin film was generated on the substrate at a deposition rate of 100-1000 .ANG./min.
Furthermore, formation of a PZT thin film by a CVD method has been reported using a material different from those in the above-described methods (Technical Papers of the Ceramics Society of Japan 99 [3] 248-250 (1991)). According to this report, dipivaloylmethanato lead (Pb (DPM).sub.2), titanium tetraisopropoxide (Ti (O--i--C.sub.3 H.sub.7).sub.4), and zirconium tetra tert-butoxide (Zr(O--t--C.sub.4 H.sub.9).sub.4) were used as the source materials of Pb, Ti, and Zr, respectively. As a substrate for depositing a PZT thin film, a monocrystal of MgO (100) was used. The temperatures for vaporization of the source materials were 165.degree. C., 50.degree. C., and 42.degree. C. for Pb, Zr and Ti, respectively. The source material of Pb was carried by N.sub.2 carrier gas at the rate of 180 ml/min. The source materials of Zr and Ti were carried out by N.sub.2 carrier gas respectively at the rate of 40 ml/min. The vapor of the source materials were mixed with O.sub.2 supplied at the rate of 40 ml/min., to be admitted into a CVD reaction chamber. In the CVD reaction chamber, a PZT thin film was formed on a MgO monocrystalline substrate heated to the temperature of 700.degree. C. under the pressure of 20 Torr. According to the above-described MOCVD method, a relatively thin ferroelectric ceramic film having a smooth surface can be formed at a higher deposition rate with controllability of the stoichiometric composition of the film.
In the above described method, epitaxial growth was carried out on a monocrystalline substrate such as MgO (100) and sapphire (0001). Such an epitaxial growth allows formation of a ceramic thin film with high crystallinity and dielectric constant on a substrate. However, the usage of a monocrystalline material for the formation of a ceramic thin film is not practical in the manufacture of an electronic device. When PZT or PLZT is applied to an electronic device, noble metal having a relatively high melting point such as platinum is used for an electrode in the capacitor structure. This electrode of noble metal is generally polycrystalline. If a ferroelectric ceramic layer is to be formed on a polycrystalline electrode directly employing the above-described method, a layer of appropriate crystallinity cannot easily be obtained. PZT and PLZT are materials indicating perovskite type crystal structure differing greatly in electric characteristic, particularly dielectric constant depending upon its crystallinity. In the above-described method, a PZT or PLZT film formed directly on a polycrystal such as platinum had poor crystallinity and low dielectric constant. When CVD method was used, excessive growth of the crystal was seen to result in large grain size, so that the morphology of the surface was degraded. A uniform and small grain size was required.
Those including Zr greater in amount than Ti in PZT is considered to be particularly suitable as the material of a capacitor insulating film of a DRAM due to the fact that it can suppress leakage current. When a PZT film is to be formed on platinum under the condition of supplying a greater amount of zirconium source in the above-described CVD in order to obtain such a Zr rich material, zirconium oxide (ZrO.sub.2) is easily precipitated. Under such conditions, a PZT of uniform perovskite crystal phase could not be obtained. This means that a PZT film of a satisfactory level of dielectric constant cannot be obtained.
In the above-described FET, a gate insulating film must be formed of a material exhibiting ferroelectricity at ordinary temperature. In accordance with a sol-gel method, sputtering method, or a CVD method, a monocrystalline substrate of sapphire (0001) and MgO (100) allows production of a PZT or PLZT film having appropriate crystallinity and exhibiting ferrolectricity at ordinary temperature. A PLZT film having crystallinity of some degree can be formed even on a platinum monocrystalline substrate. However, when a PZT film or a PLZT film is formed directly on a semiconductor substrate, particularly a Si substrate, a film cannot easily be obtained that has appropriate crystallinity and ferroelectricity. Therefore, it is difficult to apply sufficient characteristics to a device by an amorphous PZT film or PLZT film.
A PZT of appropriate crystallinity has a dielectric constant of at least 1000. This is a very high value in comparison with silicon dioxide (dielectric constant 3.8) and silicon nitride (dielectric constant 7.4) often used in current semiconductor memory devices. The potential of PZT as the capacitor dielectric material in semiconductor memory devices, particularly DRAMs of high integration density is great.
However, application of PZT to an electronic device had a problem set forth in the following. In a semiconductor device having a structure as shown in FIG. 4, a capacitor having a structure sandwiching PZT between a pair of electrodes is likely to have a leakage current density greater than practical usage level. This means that the consumption power of the device is increased.